What is the meaning of a mortgage (or pledge)? What is its ruling? And what is the evidence for it?

Pledging (Al-Rahn) linguistically means: permanence and continuity, or from confinement.

Religiously, it is: the asset that is made a security for a debt, so that the debt may be collected from its price if it is impossible to collect it from the debtor.

Pledging is permissible by the Qur'an, Sunnah, and scholarly consensus (Ijma'):

In the Qur'an, Allah Almighty says: «AND IF YOU ARE ON A JOURNEY AND DO NOT FIND A SCRIBE, THEN A PLEDGE (IS TO BE HELD) IN POSSESSION.»

In the Sunnah, the Messenger of Allah (peace be upon him) bought food from a Jew and pledged his shield to him. And the Prophet (peace be upon him) said: «The ridden animal may be ridden at its expense if it is pledged, and the milk of the milch animal may be drunk at its expense if it is pledged, and the expense is upon the one who rides and drinks.» He also said: «The pledge is not to be forfeited.»

Muslims have unanimously agreed on the general permissibility of pledging.